In the well-known TDMA technique, several earth stations share one of the transponders of a satellite to which they transmit pulse bursts which are received sequentially and retransmitted to the stations. The time is divided into recurring frames comprising a plurality of time slots, one of which is assigned to a given station. During its assigned time slot, the station transmits a pulse burst to the satellite which retransmits same to one or more stations for which it is destined. The pulse bursts transmitted by the various stations must reach the satellite transponder at specific instants to prevent overlapping and to enable the transponder to retransmit the pulse bursts serially. It is, therefore, essential that the pulse bursts from the various stations be synchronized to ensure that they are transmitted at precise instants, thereby preventing "collisions" at the transponder.
In those TDMA networks which use a satellite comprising a global beam antenna adapted to carry the down-link traffic, synchronization of the stations is usually achieved by means of the so-called single loop technique wherein a given station designated as reference station transmits a reference pulse burst which is retransmitted by the satellite transponder to all stations of the network, each of which then adjusts the time at which it is to transmit its pulse burst in accordance with the time interval between the instant at which it receives the reference pulse burst and that at which it receives from the satellite a reference word it has transmitted thereto. The single loop synchronization technique has been widely described in the literature; in this connection, reference may be made to the article by H. Ganssmantel and B. Ekstrom entitled "TDMA Synchronization for Future Multitransponder Satellite Communication," International Conference on Communications, Minneapolis, June 17-19, 1974.
In TDMA networks, single-loop synchronization can only be used with those stations which are located in the geographical area wherein their own pulse burst are retransmitted, as in the case of TDMA networks which use a satellite equipped with a global beam antenna to carry the down-link traffic. At the present time, the trend is to use satellites comprising narrow, or spot, beam directional antennas to carry such traffic. This results in improved overall system performance and is made easier by the increased precision of today's satellite stabilization techniques. In those networks which use a satellite equipped with directional beam antennae transmitting signals at different frequencies, the stations are unable to receive their own pulse bursts from the satellite and, consequently, the network cannot be synchronized using the signal-loop technique.
To synchronize a TDMA network comprising a satellite equipped with directional beam antennae transmitting signals at various frequencies, the so-called SDMA/SS-TDMA (Space Division Multiple Access/Spacecraft Switched Time Division Multiple ACCESS) technique has been developed. In this technique, synchronization of the stations is achieved by means of an absolute time reference provided by the satellite. For more details, reference should be made to the article by M. Asahara, et al. entitled "Synchronization and Acquisition in SDMA Satellite Communication System," International Conference on Communications, Minneapolis, June 17-19, 1974. Obviously, the SDMA/SS-TDMA technique requires the use of very expensive equipment carried by the satellite.
Another technique for synchronizing a TDMA network comprising a satellite equipped with directional beam antennae transmitting signals at various frequencies is described in French Pat. No. 72 46741 (publication no. 2,169,074). In that technique, the satellite includes, in addition to the various directional beam antennae which transmit signals between the various stations, a global beam antenna which performs the same function and allows the use of the single-loop synchronization technique. Obviously, the synchronization technique described in that patent requires a duplication of the satellite-borne equipment and increases the cost and the weight of the satellite.